Multiple Input Multiple Output (MIMO) technology has been widely adopted as the key technology of the evolutions of variant wireless communication standards, such as the Institute of Electrical and Electronic Engineers (IEEE) 802.11n/802.16e, the 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), the Third Generation Partnership Project 2 (3GPP2) Air Interface Enhancement (AIE) and so on. Systems with a single input and/or single output tend to be relative simple, such that adjustment of transmission scheme usually involves merely changing the transmitting power, modulation constellation and coding rate of one single data stream. By contrast, the adjustment of transmission scheme for MIMO systems is more complex due to one more spatial dimension introduced by deploying multiple antennas. Without loss of generality, a given MIMO transmission scheme may involve determining the number of multiple transmitted data streams, the modulation constellation and coding rate of overall data streams or each individual data stream, relative power and/or phase among different data streams, the mapping method between different data streams and different transmit antennas, etc. For consistency with terminology used in the art (for both single input single output systems), one MIMO transmission scheme is still referred to as a “Modulation and Coding Scheme” (MCS) in many other sources and also in this document, although a given MIMO transmission scheme can include more than only modulation and coding.
As a starting point, it should be noted that the wireless channel is a time-variant medium, such that its capacity at one point in time may be different than its capacity at another point in time. For example, due to changes in the environment, time varying multi-path fading, other nearby communications, and the like, the achievable data throughput of a wireless channel changes. In many systems, link adaptation technology is adopted to adjust the transmission scheme such that a higher transmitting data rate can be supported by the channel.
Such prior art link adaptation has been well understood for single input and/or single output systems. For example, one prior art solution is to track signal to noise ratio over a time period and to base link adaptation on a time-averaged measurement of that parameter. However, MIMO systems have vector/matrix valued channel state information such that scalar-valued signal to noise ratio is generally not precise enough to pick an optimal transmission scheme at a given moment. Moreover, the time period of one packet transmission of current broadband communication systems is so short that time-averaged channel state information is often “stale” by the time it is used by link adaptation systems.